There have conventionally been developed various switching power supply devices such as AC/DC converters and DC/DC converters. Patent Document 1 identified below discloses a DC/DC converter as described below.
The DC/DC converter includes: a transformer including a primary winding to one end of which an input voltage is applied, a secondary winding, and an auxiliary winding provided on the primary side; a switching transistor provided on the path of the primary winding; an output capacitor having the potential at its one end fixed; a diode provided between the other end of the output capacitor and one end of the auxiliary winding with its cathode on the output-capacitor side; and a control circuit (control IC) controlling the switching transistor between ON and OFF.
The control circuit includes: a power terminal connected to the other end of the output capacitor; a HIGH-voltage terminal to which the input voltage is fed; and a charge transistor provided between the HIGH-voltage terminal and the power terminal.
The control circuit further includes: a current limiting circuit that, in a first state where the voltage at the power terminal is lower than a first threshold voltage, limits the charge current passing from the HIGH-voltage terminal to the power terminal through the charge transistor and that, in a second state where the voltage at the power terminal is higher than a second threshold voltage set higher than the first threshold voltage, reduces the charge current down to substantially zero.
The DC/DC converter mentioned above operates as follows. Immediately after the power supply is started up when the voltage at the power terminal is lower than the first threshold voltage, that is, in the first state, the charge current is limited, and the supply voltage rises gently. When the voltage at the power terminal exceeds the first threshold voltage, the charge current increases, and the voltage at the power terminal rises faster. When the voltage at the power terminal exceeds the second threshold voltage, the charge current is reduced down to substantially zero. At this point, the control circuit starts up, and starts to control the switching transistor between ON and OFF. Thus, the voltage at the power terminal is stabilized by an auxiliary converter including the auxiliary winding, the diode, and the output capacitor.